1. Field of the Invention
Implementations of various technologies described herein generally relate to the field of well logging, and more particularly, to techniques in which tools equipped with antenna systems having transverse or tilted magnetic dipoles may be used for improved electromagnetic measurements of subsurface formations.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Various well logging techniques are known in the field of hydrocarbon exploration and production. These techniques typically use tools or tools equipped with sources adapted to emit energy into a subsurface formation that has been penetrated by a borehole. The emitted energy may interact with the surrounding formation to produce signals that may then be detected and measured by one or more sensors. By processing the detected signal data, a profile of the formation properties may be obtained.
Examples of well logging tools may include electromagnetic (EM) resistivity tools, such as induction and propagation tools. The EM resistivity tools may be disposed within a borehole to measure the electrical conductivity (or its inverse resistivity) of earth formations surrounding the borehole. A typical electromagnetic resistivity tool includes a transmitter antenna and one or more (typically a pair) receiver antennas disposed at a distance from the transmitter antenna along the axis of the tool.
EM induction tools measure the resistivity (or conductivity) of the formation by measuring the voltage induced in the receiver antenna as a result of magnetic flux induced by currents flowing through the formation in response to an EM signal from the emitting (or transmitter) antenna. EM propagation tools operate in a similar fashion but typically at higher frequencies than do EM induction tools for comparable antenna spacings. Propagation tools typically operate at a frequency range of 1 kHz-2 MHz, but may also operate in the gigahertz range, as in dielectric logging tools.
Conventional transmitters and receivers are antennas formed of coils having one or more turns of insulated conductor wire wound around a support. These antennas are typically operable as transmitters and/or receivers.
A coil carrying a current (e.g., a transmitter coil) is configured to generate a magnetic field. The electromagnetic energy from the transmitter antenna may be transmitted into the surrounding formation, which induces a current (eddy current) flowing in the formation around the transmitter. The eddy current in the formation in turn may generate a magnetic field that induces an electrical voltage in the receiver antennas. If a pair of spaced-apart receivers is used, the induced voltages in the two receiver antennas would have different phases and amplitudes due to geometric spreading and absorption by the surrounding formation. For conventional induction tools, the coils are wound in opposition and connected together in series. The number of turns and the location of the coils are adjusted so that the combined voltage is nearly zero in air. Older induction tool designs often had more than one transmitter and more than two receivers. For propagation tools, the phase shift, φ, and attenuation, A, between the coils may be measured. The phase shift and the attenuation from the two receivers may be used to derive electrical properties of the formation. The detected phase shift (φ) and attenuation (A) may depend not only on the spacing between the two receivers and the spacing between the transmitter and the receivers, but also on the frequency of EM waves generated by the transmitter.
In conventional EM induction and propagation logging tools, the transmitter and receiver antennas are mounted with their axes along the longitudinal axis of the tool. Thus, these tools are implemented with antennas having longitudinal magnetic dipoles (LMD). An emerging technique in the field of well logging is the use of tools including antennas having tilted or transverse coils, i.e., where the coil's axis is not parallel to the longitudinal axis of the support or borehole. These antennas generate a transverse or tilted magnetic dipole (TMD) moment.
Electromagnetic measurements made by tools such as propagation and induction tools may contain information regarding electromagnetic properties of the media through which the signal passes. Information such as distance to bed boundaries, formation dip, and anisotropy may be extracted from the received signals.